Positioning service includes navigation, assets tracing, medical care, and the like, and can capture location information of a mobile terminal quickly and accurately and enriches work and life of people. Method of positioning service generally includes global positioning system (Global Positioning System, GPS for short) positioning and wireless local area network (Wireless Local Area Network, WLAN for short) positioning. The GPS positioning depends on capture and reception of satellite signals. In an indoor environment, however, it is hard to receive signals of a positioning satellite, which leads to that the GPS positioning cannot be used normally. A WLAN is a computer local area network that uses wireless channels as transmission media. By means of a lightweight mobile device (such as a notebook computer, a palmtop computer, a personal digital assistant, and a positioning label), a WLAN positioning user may obtain high-quality access and requirement-compliant positioning services under flexible and efficient network coverage of the WLAN.
WLAN positioning methods mainly include a time of arrival (Time of Arrival, TOA for short) positioning method, a time difference of arrival (Time Difference of Arrival, TDOA for short) positioning method, and a received signal strength identifier (Received Signal Strength Identification, RSSI)-based positioning method. The RSSI positioning method mainly includes a fingerprint positioning method and a propagation model positioning method.
The basic principle of the TOA is as follows: When a terminal sends a signal, a timestamp t1 is added; when an access point (Access Point, AP for short) receives the signal, the access point records time t2 at which the signal is received. The difference between the two time values is the propagation duration of radio waves in the air. The speed of radio waves propagated in the air is the light velocity c. Therefore, the distance between the AP and the terminal is c*(t2−t1). With locations of at least three APs, the location of the terminal can be worked out according to trilateral positioning principle.
TDOA is an improved algorithm of TOA. TDOA still works by estimating the distance according to time. Unlike TOA, TDOA does not require the terminal to add a timestamp when the terminal sends a message; after at least three APs receive the message, the difference of distances from the terminal to the APs is obtained according to the time difference of receiving the signal. The difference between distances to two points is equal to a constant, which coincides with hyperbolic characteristic. With three APs, three hyperbolas are obtained, and the intersection of the three hyperbolas is the positioning location (or only two of the hyperbolas are considered).
The basic principle of the propagation model positioning method is as follows: researching a propagation model of a signal in the air, and estimating attenuation of the signal that has passed obstacles such as walls, thereby calculating strength of the signal in each location over a network.
However, the forgoing WLAN positioning methods are vulnerable to impact of reflection indoors, which affects positioning precision and leads to considerable errors in positioning results.